A typical propeller fan has a gap between the bellmouth and the blade tips. A leakage flow thus occurs from a positive pressure surface of a blade to a negative pressure surface of the blade via the gap. As illustrated in FIG. 15, for example, the leakage flow develops while moving from the leading edge toward the trailing edge of the blade and forms a blade tip vortex having a spiral shape. The blade tip vortex increases the blowing noise and raises the input of a fan motor. In FIG. 15, the reference numeral 1 represents a hub and the reference numeral 2 represents a blade. The reference numeral 2a represents a leading edge of the blade 2, the reference numeral 2b represents a trailing edge of the blade 2, and the reference numeral 2c represents a blade tip of the blade 2.
Patent Document 1 discloses a diagonal flow fan having a rib formed on the positive pressure surface of a portion of each blade tip that is not surrounded by the bellmouth. The height of the rib becomes gradually greater from the inlet side toward the outlet side. In this case, some of the air flow introduced from the inlet side is prevented from moving along the blade positive pressure surface and being blown out via the portion of the blade tip that is not surrounded by the bellmouth. This improves the air blowing performance and reduces the blowing noise. However, the technical problem that is to be solved by the invention of Patent Document 1 is basically related to a radially outward flow of a diagonal flow fan, but is not suppression of a blade tip vortex. In addition, the invention described in Patent Document 1 does not necessarily suppress the blade tip vortex effectively.
Further, in order to form the above-described rib on each blade, restriction is set on the blade in terms of the shape of the blade tip. Also, if the rib is formed on the blade, the rib influences the blowing characteristics of each blade and disadvantageously increases the weight of the blade.    Patent Document 1: Japanese Laid-Open Patent Publication No. 5-44695 (see pages 2 and 3 of Description and FIGS. 1 and 2)